As a display device for guiding image light emitted from a small-sized display element to an observer's eyeball and displaying a virtual image of the display element within the observer's field of view, a head-mountable, compact and lightweight device excellent in portability has been proposed (for example, see Patent Literature (PTL) 1). Such a display device employs a virtual image observation optical system using a light guide prism for guiding the image of the display element.
In the virtual image observation optical system using the light guide prism, part of light emitted from the display element tends to be reflected off a side surface of the light guide prism and cause a ghost image. A method of forming a groove having a V-shaped cross section (hereafter referred to as “V-shaped groove”) in the side surface of the light guide prism is known to overcome this defect. To remove the ghost by the V-shaped groove, a method of forming successive V-shaped grooves in the side surface of the prism to remove the light (hereafter referred to as “ghost light”) causing the ghost at all positions of the side surface is available (for example, see PTL 2). A method of forming a V-shaped groove only in one part of a flat side surface is known, too (for example, see PTL 3).